1. Field of the Invention
The present invention relates to a display device such as a liquid crystal display device and an EL display device.
2. Description of the Related Art
There are two methods to provide a driver circuit which drives a source line and a gate line in a display device such as a liquid crystal display device and an EL display device. One is a method by which a pixel and a driver circuit are integrated over one substrate. The other is a method by which part of or all of the function of a driver circuit is integrated into an IC chip, and the IC chip is mounted over a substrate including a pixel by a chip on glass (COG) or a chip on film (COF). A display device with a medium or larger size (10 inches or larger) is mostly mounted with a driver IC chip. This is because an amorphous silicon transistor, of which the mobility is not high enough to be employed to a driver circuit, is used for a pixel in a middle-sized display device and a large-sized display device.
The number of pins of a driver IC chip is generally 300 to 500; therefore, the number of driver IC chips depends on a resolution of the display device. When the resolution of a color display device is QVGA (320×240), the number of source lines is 320×3 (RGB)=960 and the number of gate lines is 240, so that 1 gate driver IC chip and 3 source driver IC chips are used.
FIG. 11 is a block diagram showing an example of a structure of a conventional display device (with RGB color display, resolution QVGA) in which a source driver IC chip is used. As shown in FIG. 11 and a display device 600 includes a pixel region 610 and a gate driver circuit 620. As an example, the gate driver circuit 620 is incorporated in the display device 600 and not externally provided: the gate driver circuit 620 is integrated with the pixel region 610 over a substrate 601. A source driver circuit is formed using three external components: source driver IC chips 631 to 633.
In order to reduce the number of driver IC chips without lowering resolution, the number of input terminals connected to the pin of driver IC chip needs to be reduced. For example, the number of input terminals can be reduced by providing a switch in each source line over the same substrate as the pixel region. A structural example of a display device with such a structure is shown in FIG. 13.
In a display device 650 of FIG. 13, color display is performed by an RGB method and the resolution is QVGA as in the display device 600. The display device 650 includes a source driver IC chip 651, which is externally provided, as a source driver circuit and an analog switch array 652 incorporated in the substrate 601. The analog switch array 652 includes the same number of switch circuits 660 as the number of pins to which an image data of the source driver IC chip 651 is output. When the resolution is QVGA, 320 switch circuits 660 are provided in the analog switch array 652. An example of a structure of the switch circuit 660 is shown in FIG. 14.
The switch circuit 660 includes an input terminal 661 connected to an output of the source driver IC chip 651 and 3 transistors 671 to 673 connected to the input terminal 661. Output terminals 681 to 683 in the transistors 671 to 673 are connected to different source lines. The transistors 671 to 673 function as switching elements and on/off thereof is controlled by a selection signal (SLC-R, SLC-G, and SLC-B) input from a gate. By employing the display device 650 of FIG. 13 to an EL display device, time division driving (time gray scale display) can be performed (for time division driving, see Patent Document 1, for example).
However, in the display device 650 of FIG. 13, the number of source driver IC chips 651 is 1; therefore, a period to write an image data to a source line is shortened compared to the display device 600 of FIG. 11. This will be described with reference to FIG. 12 and FIG. 15. FIG. 12 is a flow chart showing a driving method of the display device 600. FIG. 15 is a flow chart showing a driving method of the display device 650.
In FIG. 12 and FIG. 15, “G1 to G240” denote 240 gate lines. “S1R, S1G, S1B, S2R, . . . S320B” denote 320×3 source lines. “S2R” indicates a second source line to which a red image data is input.
As shown in FIG. 12, the gate lines G1, G2, G3, . . . G240 of the display device 600 are sequentially selected. In a gate line selection period, image data corresponding to a selected row is input to each source lines S1R, S1G, S1B, S2R, . . . S320B. In FIG. 12, an input signal to the source lines S1R, S1G, S1B, S2R, . . . S320B in a period during which an n-th gate line Gn is selected is shown as an example.
Next, a driving method of the display device 650 is described with reference to FIG. 15. In the display device 650, the gate lines G1, G2, G3, . . . G240 are sequentially selected. However, in the display device 650, 3 image data of RGB are output from 1 terminal of the source driver IC chip 651.
In a selection period of the gate line Gn, a selection signal SLC-R is input, whereby the transistor 671 of the switch circuit 660 of FIG. 14 is turned on, and red (R) image data which is output from the source driver IC chip 651 is output to the source lines (S1R, S2R, . . . S320R). Then, the selection signal SLC-R becomes low and the selection signal SLC-G becomes high, whereby the transistors 671 and 673 are turned off and the transistor 672 is turned on; thus, green (G) image data is output to the source lines (S1G, S2G, . . . S320G). Then, the selection signal SLC-G is low, the selection signal SLC-B is high, whereby blue (B) image data is output to the source lines (S1B, S2B, . . . S320B). In such a manner, in the display device 650, 3 image data output from 1 output terminal of the driver IC chip 651 are input to 3 source lines.